Dunbar’s Number reveals a fundamental constraint on human social organization that has profound implications for how we structure and manage groups. While we may aspire to maintain meaningful connections with hundreds or thousands of people in our increasingly connected world, our cognitive architecture appears optimized for a more modest social circle of around 150 individuals.

This limitation suggests that organizations naturally fragment when they exceed this threshold. Without deliberate intervention, large groups tend to splinter into smaller, more manageable sub-groups where members can maintain the kind of nuanced relationships that foster trust, cooperation, and effective communication; this explains why successful companies often restructure into smaller units, why military companies historically hover around 150 members, and why effective communities naturally organize into nested layers of intimacy.

The Primate Neocortex Is The Origin Story

In 1993 Robin Dunbar, a British anthropologist and evolutionary psychologist, studied brain size, group size, and language development in humans and primates in developing his “Dunbar’s Number”. Here, Dunbar suggested that the maximum group size of 150, which he found represented in natural-forming groups all over the world in different domains and cultures, was a function of neocortex size in the primate brain. The size of a primate’s neocortex, which is the part of the brain associated with cognition and language, relative to the body, was found to be linked to the optimal size of a cohesive social group.

Dunbar postulated that there was a calculable ratio between brain size and group size, and mapped this using neuroimaging and observation of time spent on important social behaviors in non-human primates. The complexity a specific social system can handle was indeed found to be limited by the ratio between brain size and group size, now called “Dunbar’s Number”.

Dunbar’s Number For Humans Throughout History

Dunbar and his colleagues next applied “Dunbar’s Number” to humans, examining historical, anthropological and contemporary psychological data on group sizes and found remarkable consistency around the number “150”. According to Dunbar and many other researchers testing his hypothesis, the magic number “150” has been observed in: early hunter-gatherer societies and tribe sizes during the Pleistocene and Neolithic eras; 11th to 18th Century towns, villages, church parishes, military units, and organizations; and a surprising array of modern groupings such as business offices, communes, clubs, factories, residential campsites, and even Christmas card lists and social media channels!

Says Dunbar on the “Rule of 150” regarding social media channels: “When people have more than 150 friends on Facebook or 150 followers on Twitter, these represent the normal outer layers of contacts: the 500 and 1500. These digital media – and I’m including telephones in there – are really just providing you with another mechanism for contacting friends.”

Dunbar’s Number With Nested Group Sizes

Although an individual’s social network may include many more people, 150 contacts marks the cognitive limit on those with whom we can maintain a stable social relationship involving trust and obligation with – move beyond that and people are mere acquaintances.

Interestingly, Dunbar also put forward a series of nested numbers of meaningful relationships for groups of various sizes. According to his theory, people may migrate in and out of these layers, but space has to be carved out for any new entrants. Dunbar isn’t sure why these layers of numbers are all multiples of five, but says, “this number five does seem to be fundamental to monkeys and apes in general”. Exceed 150, and a network is unlikely to last long or cohere well – Dunbar argues that going beyond 150 leads to a lack of cohesion and reduces the lifespan of the group as a result.

The nested groups outlined by Dunbar are as follows:

1 or 2 special friends
5 close friends
15 good friends
50 friends
150 meaningful relationships
500 acquaintances
1500 people you can recognize

Dunbar's Number illustrated with circles of hierarchical acquaintanceship shows human's can maintain two special, five close, fifteen good, fifty friend, and one-hundred fifty meaningful relationships

Final Thoughts

Understanding Dunbar’s Number doesn’t mean we should rigidly cap all groups at 150 members. Rather, it highlights the importance of designing social structures that work with, rather than against, our cognitive limitations. Large organizations can thrive by creating smaller, interconnected teams where meaningful relationships can flourish. Digital tools can help us manage broader networks, but they cannot replace the deep, multifaceted connections that form the bedrock of human cooperation.

Ultimately, Dunbar’s Number reminds us that despite our technological sophistication, we remain fundamentally social primates with finite capacity for meaningful relationships. The most effective groups are those that acknowledge this reality and structure themselves accordingly—building strong cores of genuine connection while accepting that our ability to truly know and trust others has natural boundaries. In an era of limitless digital connections, this insight is perhaps more valuable than ever.

Thanks for reading!